Fix whitespace in fish shellrc template (#686)

## Summary
Otherwise we get a line like:
```
# Begin Devbox Post-init Hookexport PATH="$DEVBOX_PATH_PREPEND:$PATH"
```
which is obviously not what we want.

## How was it tested?
```
SHELL=fish ./devbox shell
> echo $PATH
```

Author: ipince

internal/nix/shellrc_fish.tmpl

@@ -31,9 +31,9 @@ add string-splitting logic here nor parametrize computeNixEnv based on the shell
 used. So here we (ab)use the fact that using "export" ahead of the variable definition
 makes fish do exactly what we want and behave in the same way as other shells.
 */ -}}
-{{ if .UnifiedEnv -}}
+{{ if .UnifiedEnv }}
 export PATH="$DEVBOX_PATH_PREPEND:$PATH"
-{{- else -}}
+{{- else }}
 export PATH="{{ .PathPrepend }}:$PATH"
 {{- end }}