root.bib: Fix 'μ' -> '$\mu$' in jaschke95 abstract

pdflatex doesn't deal with non-ASCII input very gracefully.  Not that
I'm rendering the abstracts, but still ;).

diff --git a/src/root.bib b/src/root.bib
index f1b764dc5847571a8d822705df694f8ed471058a..7a65ef109a14a1e54dc08c78e2946292f01db630 100644 (file)
--- a/src/root.bib
+++ b/src/root.bib
@@ -2212,9 +2212,9 @@
     mirror no additional equipment is required since the fixed
     wavelength of the laser diode is used as a calibration
     standard. The calibration is appliable in the range between
-    several ten nm and several μm. Besides vertical calibration many
-    problems of piezo elements like hysteresis, nonlinearity, creep,
-    derating, etc. and their dependence on scan parameters or
+    several ten nm and several $\mu$m. Besides vertical calibration
+    many problems of piezo elements like hysteresis, nonlinearity,
+    creep, derating, etc. and their dependence on scan parameters or
     temperature can be investigated.},
 }