docs: fix formatting and punctuation (#2320)

Author: vaeng

exercises/concept/squeaky-clean/.docs/instructions.md

@@ -1,18 +1,16 @@
 # Instructions
 
-In this exercise you will implement a partial set of utility routines to help a developer
-clean up identifier names.
-
-In the 5 tasks you will gradually build up the routine `Clean` A valid identifier comprises
-zero or more letters and underscores.
-
-In all cases the input string is guaranteed to be non-null. If an empty string is passed to the `Clean` function, an empty string should be returned.
-
+In this exercise, you will implement a partial set of utility routines to help a developer clean up identifier names.
+In the 5 tasks you will gradually build up the routine `Clean`.
+A valid identifier comprises zero or more letters and underscores.
+In all cases, the input string is guaranteed to be non-null.
+If an empty string is passed to the `Clean` function, an empty string should be returned.
 Note that the caller should avoid calling the routine `Clean` with an empty identifier since such identifiers are ineffectual.
 
 ## 1. Replace any spaces encountered with underscores
 
-Implement the (_static_) `Identifier.Clean()` method to replace any spaces with underscores. This also applies to leading and trailing spaces.
+Implement the (_static_) `Identifier.Clean()` method to replace any spaces with underscores.
+This also applies to leading and trailing spaces.
 
 ```csharp
 Identifier.Clean("my   Id");