Fix docs for static methods

The three static operations, clone/open/init showed examples using an
instance grgit instead of calling via the class Grgit. This can
mislead users and result in NPEs if they're trying to use it with
org.ajoberstar.grgit for a project without a git repo.

Fixes #372

docs/modules/ROOT/pages/grgit-clone.adoc

@@ -8,14 +8,14 @@ grgit-clone - Clone a repository into a new directory
 
 [source, groovy]
 ----
-grgit.clone(dir: <path>, uri: <path or uri>, remote: <name>, all: <boolean>,
+Grgit.clone(dir: <path>, uri: <path or uri>, remote: <name>, all: <boolean>,
   bare: <boolean>, branches: <full refs>, checkout: <boolean>,
   refToCheckout: <name>, credentials: <credentials>)
 ----
 
 [source, groovy]
 ----
-grgit.clone {
+Grgit.clone {
   dir = <path>
   uri = <path or uri>
   remote = <name>

docs/modules/ROOT/pages/grgit-init.adoc

@@ -8,12 +8,12 @@ grgit-init - Create an empty Git repository
 
 [source, groovy]
 ----
-grgit.init(dir: <path>, bare: <boolean>)
+Grgit.init(dir: <path>, bare: <boolean>)
 ----
 
 [source, groovy]
 ----
-grgit.init {
+Grgit.init {
   dir = <path>
   bare = <boolean>
 }

docs/modules/ROOT/pages/grgit-open.adoc

@@ -8,17 +8,17 @@ grgit-open - Open an existing Git repository
 
 [source, groovy]
 ----
-grgit.open()
+Grgit.open()
 ----
 
 [source, groovy]
 ----
-grgit.open(dir: <path>, currentDir: <path>, credentials: <credentials>)
+Grgit.open(dir: <path>, currentDir: <path>, credentials: <credentials>)
 ----
 
 [source, groovy]
 ----
-grgit.open {
+Grgit.open {
   dir = <path>
   currentDir = <path>
   credentials = <credentals>