Fix a typo at ȳ(547 nm) in CIE 1931 2° observer table

The tabulated value was 0.9874812.
Per CIE 018:2019 Table 6 (DOI 10.25039/CIE.DS.xvudnb9b), the correct
value is 0.9874182.  Verification from CIE published data:

```
import urllib.request, csv, io

url = "https://files.cie.co.at/CIE_xyz_1931_2deg.csv"
data = urllib.request.urlopen(url).read().decode()

for row in csv.reader(io.StringIO(data)):
    if row and row[0].strip() == "547":
        print(f"y-bar at 547nm = {row[2]}")
        break
```

Signed-off-by: Jonathan Swartz <jonathan@jswartz.info>

Author: swahtz

src/pbrt/util/spectrum.cpp

@@ -403,7 +403,7 @@ const Float CIE_Y[nCIESamples] = {
     0.8620000,       0.8738108,       0.8849624,       0.8954936,       0.9054432,
     0.9148501,       0.9237348,       0.9320924,       0.9399226,       0.9472252,
     0.9540000,       0.9602561,       0.9660074,       0.9712606,       0.9760225,
-    0.9803000,       0.9840924,       0.9874812,       0.9903128,       0.9928116,
+    0.9803000,       0.9840924,       0.9874182,       0.9903128,       0.9928116,
     0.9949501,       0.9967108,       0.9980983,       0.9991120,       0.9997482,
     1.0000000,       0.9998567,       0.9993046,       0.9983255,       0.9968987,
     0.9950000,       0.9926005,       0.9897426,       0.9864444,       0.9827241,