[expr.prim.req] Repair meaningless "A requirement is one that [..]" phrasing.

Author: zygoloid

source/expressions.tex

@@ -1514,8 +1514,8 @@
 
 \pnum
 A \grammarterm{requires-expression} provides a concise way to express 
-requirements on template arguments. 
-A requirement is one that can be checked by name lookup~(\ref{basic.lookup})
+requirements on template arguments
+that can be checked by name lookup~(\ref{basic.lookup})
 or by checking properties of types and expressions.
 
 \begin{bnf}