[class.conv.ctor] Add a space (#6497)

Author: Eisenwave

source/classes.tex

@@ -2351,7 +2351,7 @@
 \begin{codeblock}
 struct X {
     X(int);
-    X(const char*, int =0);
+    X(const char*, int = 0);
     X(int, int);
 };