Fix code typo in ghc-debug-client example

The `ghc-debug-client` example refers to an out-of-scope variable `h`, which I
believe is meant to refer to `g` instead.

README.md

@@ -47,7 +47,7 @@ p1 e = do
         precacheBlocks
         (r:_) <- gcRoots
         buildHeapGraph (Just 10) r
-  putStrLn (ppHeapGraph (const "") h)
+  putStrLn (ppHeapGraph (const "") g)
 ```
 
 The API for writing debuggers is described in the `GHC.Debug.Client` module.

client/src/GHC/Debug/Client.hs

@@ -12,7 +12,7 @@ p1 e = do
         precacheBlocks
         (r:_) <- gcRoots
         buildHeapGraph (Just 10) r
-  putStrLn (ppHeapGraph (const "") h)
+  putStrLn (ppHeapGraph (const "") g)
 @
 
 -}

docs/index.md

@@ -45,7 +45,7 @@ p1 e = do
         precacheBlocks
         (r:_) <- gcRoots
         buildHeapGraph (Just 10) r
-  putStrLn (ppHeapGraph (const "") h)
+  putStrLn (ppHeapGraph (const "") g)
 ```
 
 The API for writing debuggers is described in the `GHC.Debug.Client` module.