[ios.base.storage] Fix grammar typo

Author: jensmaurer

source/iostreams.tex

@@ -1458,7 +1458,7 @@
 \tcode{iarray} as necessary to include the element
 \tcode{iarray[idx]}.
 Each newly allocated element of the array is initialized to zero.
-The reference returned is invalid after any other operations on the
+The reference returned is invalid after any other operation on the
 object.
 \begin{footnote}
 An implementation is free to implement both the integer
@@ -1515,7 +1515,7 @@
 \tcode{parray[idx]}.
 Each newly allocated element of the array is initialized to a null
 pointer.
-The reference returned is invalid after any other operations on the
+The reference returned is invalid after any other operation on the
 object.
 However, the value of the storage referred to is retained, so
 that until the next call to