Update templates.md

[ebarquero85]

fix bug in the example

[aldas]

This has been changed back and forth in following PRs:

• Fix template name in template.md #195
• Update templates.md #158
• Use latest version #144
• Update templates.md #148

in that exaple the template fragment is defined as {{define "hello"}}Hello, {{.}}!{{end}} meaning named as hello. I do not understand without trying atm where that need for hello.html comes from.