Documentation for filepath headers has too many backticks

[martinthomas]

Minor defect in documentation:
The example for FLASKS3_FILEPATH_HEADERS encloses the header list in back-ticks and makes it a string when it should be a dictionary as seen in the comment above.

Wrong (from docs):
{r'.txt$': {‘Texted-Up-By’: ‘Mister Foo’} }

Right:
{r'.txt$': {‘Texted-Up-By’: ‘Mister Foo’}}