[iterators.common] fix typo and add "is true" to Preconditions: clauses (#4382)

Author: timsong-cpp

source/iterators.tex

@@ -4880,7 +4880,7 @@
 \begin{itemdescr}
 \pnum
 \expects
-\tcode{holds_alternative<I>(v_)}.
+\tcode{holds_alternative<I>(v_)} is \tcode{true}.
 
 \pnum
 \effects
@@ -4905,7 +4905,7 @@
 
 \pnum
 \expects
-\tcode{holds_alternative<I>(v_)}.
+\tcode{holds_alternative<I>(v_)} is \tcode{true}.
 
 \pnum
 \effects
@@ -4950,7 +4950,7 @@
 \begin{itemdescr}
 \pnum
 \expects
-\tcode{holds_alternative<I>(v_)}.
+\tcode{holds_alternative<I>(v_)} is \tcode{true}.
 
 \pnum
 \effects
@@ -4969,7 +4969,7 @@
 \begin{itemdescr}
 \pnum
 \expects
-\tcode{holds_alternative<I>(v_)}.
+\tcode{holds_alternative<I>(v_)} is \tcode{true}.
 
 \pnum
 \effects
@@ -5059,7 +5059,7 @@
 \begin{itemdescr}
 \pnum
 \expects
-\tcode{holds_alternative<I>(v_)}.
+\tcode{holds_alternative<I>(i.v_)} is \tcode{true}.
 
 \pnum
 \effects