DOC: Cleanup CoW dev docs

doc/source/development/copy_on_write.rst

@@ -14,15 +14,15 @@ behaves as a copy.
 Reference tracking
 ------------------
 
-To be able to determine, if we have to make a copy when writing into a DataFrame,
-we have to be aware, if the values are shared with another DataFrame. pandas
+To be able to determine if we have to make a copy when writing into a DataFrame,
+we have to be aware if the values are shared with another DataFrame. pandas
 keeps track of all ``Blocks`` that share values with another block internally to
 be able to tell when a copy needs to be triggered. The reference tracking
 mechanism is implemented on the Block level.
 
 We use a custom reference tracker object, ``BlockValuesRefs``, that keeps
 track of every block, whose values share memory with each other. The reference
-is held through a weak-reference. Every two blocks that share some memory should
+is held through a weak-reference. Every pair of blocks that share some memory should
 point to the same ``BlockValuesRefs`` object. If one block goes out of
 scope, the reference to this block dies. As a consequence, the reference tracker
 object always knows how many blocks are alive and share memory.