Fix markdown in schema docs (#346)

See:
https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/organizing-information-with-tables#creating-a-table.
Fixes #339.

I have:
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- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
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docs/schema.md

@@ -46,7 +46,7 @@ This can now be invoked upon `go generate` via `//go:generate yourpkg/generate`.
 GraphQL [defines][spec#scalar] five standard scalar types, which genqlient automatically maps to the following Go types:
 
 | GraphQL type | Go type   |
-----------------------------
+|--------------|-----------|
 | `Int`        | `int`     |
 | `Float`      | `float64` |
 | `String`     | `string`  |