present: correct documented time for dates

Dates without an explicit time are parsed as 11am UTC, but the package
documentation incorrectly states that they are interpreted as 10am UTC.

Update the documentation to match the existing implementation.

Change-Id: I14f9ad788c317eded02d1a12b21ea5616fda5db5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/805960
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Cherry Mui <cherryyz@google.com>

present/doc.go

diff --git a/present/doc.go b/present/doc.go
index 2c88fb9..b43be0b 100644
--- a/present/doc.go
+++ b/present/doc.go
@@ -29,7 +29,7 @@
 
 	2 Jan 2006
 
-In this case, the time will be interpreted as 10am UTC on that date.
+In this case, the time will be interpreted as 11am UTC on that date.
 
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