Correct syntax. '&' was the original idea for '+=' that wasn't used

spec/src/main/asciidoc/ELSpec.adoc

diff --git a/spec/src/main/asciidoc/ELSpec.adoc b/spec/src/main/asciidoc/ELSpec.adoc
index 8b3be95..b447ff0 100644
--- a/spec/src/main/asciidoc/ELSpec.adoc
+++ b/spec/src/main/asciidoc/ELSpec.adoc
@@ -1831,7 +1831,7 @@
 
 /*
  * Concatenation
- * For '&', then Math()
+ * For '+=', then Math()
  */
 void Concatenation() : {}
 {