gopls/internal/lsp/source: rename FileHandle.Read to Content

It does not call the read(2) system call.
This has confused us too many times.

Change-Id: I2bd14aff70e8608ba6254f43c9ccb3bd80618b0b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/474095
Run-TryBot: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/gopls/internal/lsp/cache/snapshot.go b/gopls/internal/lsp/cache/snapshot.go
index f81c54a..0383ac8 100644
--- a/gopls/internal/lsp/cache/snapshot.go
+++ b/gopls/internal/lsp/cache/snapshot.go
@@ -535,7 +535,7 @@
 		if err != nil {
 			return "", nil, cleanup, err
 		}
-		modContent, err = modFH.Read()
+		modContent, err = modFH.Content()
 		if err != nil {
 			return "", nil, cleanup, err
 		}