Fix minor spelling error 'grade' -> 'gradle'.

Resolves gh-32.

Author: msecrist

spring-geode-docs/src/docs/asciidoc/index.adoc

@@ -144,7 +144,7 @@ To use Spring Boot for Apache Geode, declare the `spring-geode-starter` on your
 </dependencies
 ----
 
-.Grade
+.Gradle
 [source,java]
 [subs="verbatim,attributes"]
 ----