Fix angle brackets for concept

Should be "models invocable" not "models invocable<>"

Author: steve-downey

papers/P2988/new-optional.tex

@@ -953,7 +953,7 @@
 \begin{itemdescr}
 \pnum
 \constraints
-\tcode{F} models \tcode{\libconcept{invocable}<>} and
+\tcode{F} models \tcode{\libconcept{invocable}} and
 \tcode{T} models \libconcept{copy_constructible}.
 
 \pnum
@@ -980,7 +980,7 @@
 \begin{itemdescr}
 \pnum
 \constraints
-\tcode{F} models \tcode{\libconcept{invocable}<>} and
+\tcode{F} models \tcode{\libconcept{invocable}} and
 \tcode{T} models \libconcept{move_constructible}.
 
 \pnum
@@ -1499,7 +1499,7 @@
 \begin{itemdescr}
   \pnum
   \constraints
-  \tcode{F} models \tcode{\libconcept{invocable}<>}.
+  \tcode{F} models \tcode{\libconcept{invocable}}.
 
   \pnum
   \mandates