Update doc/cprover-manual/goto-analyzer.md

Co-authored-by: Peter Schrammel <[email protected]>

Author: martin-cs

doc/cprover-manual/goto-analyzer.md

@@ -301,7 +301,7 @@ variable of union type.
 `--vsd-data-dependencies`
 : Wraps each abstract object with a set of locations whether the
 variable was last modified.  The set is reset when the variable is
-written and takes the union of the two side's sets on merge.  This was
+written and takes the union of the two sides' sets on merge.  This was
 originally intended for `--dependence-graph-vs` but has proved useful
 for `--vsd` as well.  This is not strictly necessary for
 `--three-way-merge` as the mechanism it uses to work out which