Fix exception when using Pipeline Snippet Generator

The getter for credentialsId should be called getCredentialsId, not
getcredentialsId. Not having the expected getter name causes an
exception:

java.lang.UnsupportedOperationException: no public field credentialsId (or getter method) found in class stashpullrequestbuilder.stashpullrequestbuilder.StashBuildTrigger
        at org.jenkinsci.plugins.structs.describable.DescribableParameter.getValue(DescribableParameter.java:161)

Author: Guy Veraghtert

src/main/java/stashpullrequestbuilder/stashpullrequestbuilder/StashBuildTrigger.java

@@ -137,7 +137,7 @@ public String getCron() {
   }
 
   // Needed for Jelly Config
-  public String getcredentialsId() {
+  public String getCredentialsId() {
     return this.credentialsId;
   }