fix a couple of typos, grammar, and removal of whitespace

doc/manual/types.txt

@@ -390,7 +390,7 @@ string from a cstring:
 
 .. code-block:: nim
   var str: string = "Hello!"
-  var cstr: cstring = s
+  var cstr: cstring = str
   var newstr: string = $cstr
 
 

doc/tut2.txt

@@ -35,7 +35,7 @@ Object Oriented Programming
 ===========================
 
 While Nim's support for object oriented programming (OOP) is minimalistic,
-powerful OOP technics can be used. OOP is seen as *one* way to design a
+powerful OOP techniques can be used. OOP is seen as *one* way to design a
 program, not *the only* way. Often a procedural approach leads to simpler
 and more efficient code. In particular, prefering composition over inheritance
 is often the better design.

web/index.txt

@@ -42,7 +42,7 @@ Nim is efficient
 Nim is expressive
 =================
 
-* **The Nim compiler and all of the standard library are implemented in 
+* **The Nim compiler and all of the standard libraries are implemented in
   Nim.**
 * Built-in high level datatypes: strings, sets, sequences, etc.
 * Modern type system with local type inference, tuples, variants,