Add "it" to operations doc for clarity (#4957)

Author: awiteck

docs/src/operations.md

@@ -31,7 +31,7 @@ set!(c, periodic_but_decaying)
     └── max=0.428882, min=-0.428882, mean=1.04083e-17
 ```
 
-An `AbstractOperation` (or _operation_ for short) differs from a `Field` in that only represents a computation.
+An `AbstractOperation` (or _operation_ for short) differs from a `Field` in that it only represents a computation.
 Unlike `Field`s, `AbstractOperation`s do not have any data, and are associated only with minimal memory allocation.
 `AbstractOperations` are generated by inflicting `Field`s with ordinary arithmetic expressions,