Fix typos and improve grammar in stdlib readmes (#2527)

* Fix typos in the main ReadMe.md

* Fix typos in the stdlib samples ReadMe.md

Co-Authored-By: ilya-g <ilya.gorbunov@jetbrains.com>

libraries/stdlib/samples/ReadMe.md

@@ -2,14 +2,14 @@
 
 This project contains samples for the standard library functions. 
 They are located in the [test](test) source root and each sample is written like a small unit test.
-Then these samples are referenced in the documentation of the standard library functions 
-with the [`@sample`](https://kotlinlang.org/docs/reference/kotlin-doc.html#block-tags) tag and are embedded in the generated documentation as runnable samples.
+These samples are referenced in the documentation of the standard library functions 
+with the [`@sample`](https://kotlinlang.org/docs/reference/kotlin-doc.html#block-tags) tag and embedded in the generated documentation as runnable samples.
 
 
 ### Guides for sample authoring
 
-Note that samples, even though they are written as unit tests, are not conceptually the same as unit tests. 
-Unlike a unit test, which usually explores corner cases of a function being tested, 
+Note that samples, although written as unit tests, are conceptually different from them. 
+While a unit test usually explores corner cases of a function being tested, 
 the goal of a sample is to show a common use case of a function.
 
 Please see the existing samples for an inspiration on authoring new ones.
@@ -26,7 +26,7 @@ in the template, and then all specializations should be regenerated. See [the st
 depending on the specialization. 
 
 - Each sample should be self contained, but you can introduce local classes and functions in it.
-Do not use external references, other than the Standard Library itself and JDK.
+Do not use external references other than the Standard Library itself and JDK.
 
 - Use only the following subset of assertions:
 
@@ -34,7 +34,7 @@ Do not use external references, other than the Standard Library itself and JDK.
     - `assertTrue`/`assertFalse` to show a boolean value,
     - `assertFails` / `assertFailsWith` to show that some invocation will fail.
   
-  When a sample is compiled and run during the build these assertions work as usual test assertions.
+  When a sample is compiled and run during the build, these assertions work as usual test assertions.
   When the sample is transformed to be embedded in docs, these assertions are either replaced with `println` with the comment showing its 
   expected output, or commented out with `//` — this is used for `assertFails` / `assertFailsWith` to prevent execution of its failing block 
   of code.