fix typo (missing space) that messed up rst rendering (#13217)

Author: sappelhoff

mne/time_frequency/spectrum.py

@@ -1093,7 +1093,7 @@ class Spectrum(BaseSpectrum):
         Frequencies at which the amplitude, power, or fourier coefficients
         have been computed.
     %(info_not_none)s
-    method : ``'welch'``| ``'multitaper'``
+    method : ``'welch'`` | ``'multitaper'``
         The method used to compute the spectrum.
     nave : int | None
         The number of trials averaged together when generating the spectrum. ``None``